Synthesis of large numbers of different oligonucleotide sequences on solid supports has been described previously, see for example, Fodor et al., Science 251(4995), 767-73, 1991, Fodor et al., Nature 364(6437), 555-6, 1993 and Pease et al. PNAS USA 91(11), 5022-6, 1994 and U.S. Pat. No. 5,445,934.